1. Field of Invention
The present invention relates to an apparatus for producing oxidized filaments which are used for example as fire-proof fibers, as reinforcing fibers in slate or concrete board instead of asbestos fibers or as precursor filaments for producing carbon filaments or graphite filaments.
2. Description of Prior Art
It is well known that precursor filaments such as polyacrylonitrile filaments are oxidized by passing through a hot oxidizing atmosphere such as air having a temperature in the range of from about 200.degree. C. to about 300.degree. C. in a furnace. A furnace having a series of guide rollers for guiding the filaments in the furnace is known. One group of the guide rollers is provided at a lower portion and another group of the guide rollers is provided at an upper portion and the filaments are guided along the guide rollers with an up and down path. And further among such furnaces, it is known that there is a type of furnace that the lower guide rollers are provided at the outside of the bottom wall of the furnace and the upper guide rollers are provided at the outside of the top wall of the furnace and another type of furnace where the lower guide rollers are provided at the outside of the bottom wall of the furnace and the upper guide rollers are provided at the upper portion of the inside of the furnace. For example, the former type of furnace is disclosed by Japanese Patent Publication No. SHO 54-1815 and the latter type of furnace is disclosed by Japanese Patent Publication No. SHO 54-1814. Each furnace of these types has slits for passing the filaments through at the bottom wall and further the former type furnace has slits for passing the filaments through at the top wall. In these apparatus, the temperature of the atmosphere is high, since the funnel effect due to a temperature difference between the external air and the internal gas causes a suction of room air having a low temperature through the slits provided at the bottom wall of the furnace or an outblast of hot gas through the slit provided at the top wall of the furnace.
Such an effect leads to various troubles such as variance in the physical properties of filaments produced, poor working conditions and decreased efficiency due to the leakage of hot gas, on account of the temperature variations within the furnace.
To eliminate these troubles, Japanese Patent Publication No. SHO 54-1815 discloses a sealing method in which seal chambers are provided at the top and bottom in the furnace and the slits provided in the top wall and the bottom wall of the furnace are sealed by providing the upper seal chambers with a gas and by drawing out the gas in the bottom seal chambers. This approach is successful to a certain extent in solving the above problems, but it is not enough to reduce the temperature variations within the furnace and to assure the necessary sealability.
Meanwhile, it is vitally important to ensure a uniform of gas stream velocity for the purpose of maintaining a constant temperature within the furnace. As a matter of fact, however, it is difficult to maintain a constant velocity of gas stream to which individual filaments are exposed within the furnace. For instance, it is common practice in designing the oxidizing furnace used for the oxidation of precursor filaments that for the purpose of circulating a hot gas through the furnace, the hot gas outlet provided at one end of the furnace and the hot gas inlet provided at the other end of the furnace are connected outside of the furnace by a gas circulation duct via a circulating fan and a heater. The stream coming into the furnace through the hot gas inlet, however, tends to be disturbed, particularly by the configuration near the gas inlet which makes uneven the stream velocities to which the filaments are exposed. This also tends to make uneven the temperature of the atmosphere to which individual groups of filaments are exposed in the furnace, resulting in a wide variance in the product qualities, and in extreme cases, resulting in a breakage of filaments, which causes a disruption of the continuous operation.
When the filaments are broken, the remedy will be difficult and when a multi-stage heat treatment is conducted with a plurality of furnaces, the remedy will be extremely difficult.